The present invention relates to a process for making a semiconductor device, including at least a first predeposition and diffusion drive-in step of impurity material of one conductivity type in a substrate of another conductivity type so as to form a well of said one conductivity type therein, and a second predeposition and diffusion drive-in step of impurity material so as to form a region separated from the well boundary by a channel zone.
Such a process is already known from the article "A Monolithic 200-V CMOS Analog Switch" by J. D. Plummer and J. D. Meindl, published in the IEEE Journal of Solid-State Circuits, Vol. SC-11, No. 6, December 1976, pp. 809-817. It is used for making a high-voltage N-channel DMOS transistor by forming the well and the region through a same opening in an oxide layer covering the substrate. The well, the region and the substrate are made of P.sup.-, N.sup.+, and N.sup.- material respectively. As mentioned in this article the channel between the N.sup.+ region and the N.sup.- substrate which are used as the source and the drain of the DMOS transistor should at least have a length of 2.5 micron in order to prevent punchthrough breakdown between drain and source.
It has been found that such a punchthrough breakdown is relatively frequent.